The present invention relates to a modified process for selectively hydrogenating acetylene contained in hydrogen chloride gase. More specifically, the present invention also pertains to a modified process for producing 1,2-dichloroethane by subjecting the hydrogen chloride gas mentioned above to oxychlorination reaction of ethylene.
In the commerical production of vinyl chloride, 1,2-dichloroethane is decomposed at an elevated temperature so as to form vinyl chloride and hydrogen chloride. The hydrogen chloride is subsequently separated from the vinyl chloride to be subjected to an oxychlorination reaction together with ethylene, and oxygen or air in order to produce 1,2-dichloroethane economically. In this recycling system, hydrogen chloride gas, resulting from the thermal decomposition of 1,2-dichloroethane, ordinarily contains a few thousands parts per million (ppm) of acetylene, which is converted into dichloroethylene, trichloroethylene, tetrachloroethane and others in the oxychlorination reaction; as a result, these products diminish the quality of the produced 1,2-dichloroethane. Particularly, trichloroethylene is so difficult to remove from 1,2-dichloroethane by distillation that it brings about the thermal decomposition of 1,2-dichloroethane, which becomes the cause of delay in the reaction of generating vinyl chloride. Therefore, acetylene, when contained in hydrogen chloride gas, wastefully consumes hydrogen chloride in the oxychlorination reaction, and deteriorates the purity of the produced 1,2-dichloroethane.
Because acetylene is difficult to separate from hydrogen chloride by means of a simple physical operation such as distillation, chemical processes have been applied to separate them from one another. For example, there is a process disclosed in Japanese Laid Open Pat. No. 11898/1979 (Dow Chemical) by which acetylene is converted into chlorinated derivatives being heated to 300.degree.-500.degree. C. in the presence of oxygen. According to this process, not only is hydrogen chloride consumed wastefully but also the reaction needs to be conducted at an elevated temperature; consequently, this process is not preferable economically and brings about corrosion of the reaction equipment.
In contrast thereto, a conversion process of acetylene into ethylene can be considered to save the wasteful consumption of hydrogen chloride and contribute to the improvement of the reaction so that this process must be more advantageous than the former. In regard to this, a process has been publicly known, which utilizes palladium or platinum supported on alumina or silica (Japanese Patent Publication No. 9522/1968, Goodrich). According to the process, the reaction is conducted according to the following conditions: temperature, 125.degree.-175.degree. C.; molar ratio of hydrogen to acetylene, (1-6.6):1; space velocity (volume of gas current/volume of catalyst/time), 2,000-3,000. To be concrete, the palladium catalyst supported on alumina carrier ordinarily requires a space velocity of about 2,000 hr.sup.-1. Additionally, it has been also said about the process that not only is much catalyst needed because of the low activity but also the selectivity of ethylene is as low as about 50%. In the case of the palladium catalyst supported on highly pure silica carrier with specific surface area of 3 m.sup.2 /g or less as in West Germany Laid Open Pat. No. 2438153 (Degussa), it has been said that, although the selectivity of ethylene is 60-70% and the life of the catalyst lasts more than one year, the process is still economically not so desirable, when taking it into account that the space velocity is limited to about 2,000 hr.sup.-1, and much catalyst is wastefully consumed.